Method for fractionating air by liquefaction and rectification



March 12, 1957 J. WUCHERER ETAL METHOD FOR FRACTIONATING AIR BYLIQUEFACTION AND RECTIF'ICATION Original Filed Jan. 2, 1953 11vvE/vTaAS. JOHANNES WucHEeE/e n r Tom 15K United States Patent METHOD FORFRACTIONATING AIR BY LIQUE- FACTION AND RECTIFICATION Johannes Wucherer,Pullach, near Munich, and Rudolf Becker, Munich-Solln, Germany,assignors to Lincles Eismaschinen Aktiengesellschaft,Hoellriegelskreuth, near Munich, Bavaria, Germany, a corporation ofGermany Original application January 2, 1953, Serial No. 329,378,

now Patent No. 2,712,738, dated July 12, 1955. Divided and thisapplication June 3, 1955, Serial No. 513,006

Claims priority, application Germany January 10, 1952 5 Claims. (Cl.62-1755) The present invention relates to an improved method and meansfor fractionating air by liquefaction and rectification.

The present application is a division of our copending applicationSerial No. 329,378, filed January 2, 1953 now Patent No. 2,712,738.

The oxygen obtained from atmospheric air in conventional systems isusually without pressure. There are many cases, however, in which theoxygen is required at an increased pressure and must be compressed afterit has been separated. Compression cannot be effected in conventional,lubricated compressors, requiring special measures which make itdesirable that the oxygen be obtained already at an increased pressure.

A method has been proposed for obtaining oxygen by liquefaction andrectification of air. In this method the rectifying column is operatedat normal pressure, the separated oxygen being compressed in liquidstate and thereupon evaporated and heated to the ambient temperature byheat exchange with air to be fractionated. Thereby, the oxygen can beevaporated under pressure and its cold can be recovered by heat exchangewith air whose pressure is so high that it is liquefied at a temperaturewhich is higher than the evaporation temperature of the oxygen. Thismethod involves undesired losses. in order to avoid loss of cold, thedifference between the temperature of the air and that of the oxygenmust be small at the warm end of the heat exchanger. The air must bewarmer than the oxygen at each point of the heat exchanger to effectheat exchange. Except at the warm end, the temperature difference issmallest at the point of the heat exchanger where the oxygen begins toevaporate. Considering the smallest permissible temperature differenceat the warm end of the heat exchanger and at the point where evaporationof the oxygen begins and considering the velocity of the flowing mediaas well as the size of the heat exchange surfaces, the amount of air canbe calculated which must exchange heat with a given quantity of oxygen.If the amounts of air at a given pressure of the oxygen, are plotted inrelation to increasing air pressures, the curve will have a flatminimum; the amount of air condensed during the heat exchange with theoxgyen is always greater than the amount oi oxygen; at theaforementioned minimum, the amount of air is in most cases to greaterthan the amount of oxygen.

Since the total amount of the substances leaving the plant is equal tothe amount of air which enters the plant, the amount of air whichexchanges heat with the nitro gen is always smaller than the amount ofnitrogen leaving the plant, increasing loss of cold. The conditions aremore favorable in regenerators usually employed for the heat exchangebetween air and nitrogen, the pressure of the air being about equal tothat in the prefractionating "ice column, i. e., 5 to 6 atmospheresabsolute. These heat exchangers are subject to similar limitations withrespect to the relative amounts of the heat exchanging gases as theoxygen heat exchanger; in these cases, however, the amount of air may besomewhat-up to 2.5 %-smaller than the amount of nitrogen; if thedifference is greater. the heat exchange is impaired, causing additionalcold loss. if 20 parts of oxygen are obtained by the decomposition of100 parts of air, 20 l.25=25 parts of air must exchange heat with theoxygen. This leaves only parts of air for exchanging heat with parts ofnitrogen, whereas 8'O 0.975=78 parts of air would be required. Economicoperation is therefore not possible.

Economic operation can be effected only by increasing the pressure of apart of the total amount of air, whereby the specific heat is increasedand heat can be exchanged with a greater amount of nitrogen and theaforementioned gap can be closed.

This solution of the problem, however, requires that a relatively largeportion of the total air-about 35% to 55%must be compressed, whichrequired additional energy. It is not advisable to use regenerators orsimilar self-cleaning heat exchangers for highly compressed air, becausethe air remaining and lost in these devices at every reversal of fiow istoo great. If counterflow heat exchangers are used, the relatively greatamount of air must be dried and the carbon dioxide must be separatedtherefrom, involving additional cost.

It has been proposed to increase the pressure of the total amount of aironly to that pressure at which it is fractionated and to employ anitrogen cycle in which the pressure of the nitrogen is increased andthe latter exchanges heat with the oxygen evaporating at increasedpressure. In this way drying and separation of CO2 can be avoided, ifthe low pressure air heat. exchangers (regenerators) are so constructedthat a certain amount of nitrogen does not contact the heat exchangesurfaces which are coated with water ice and solid carbon dioxide, thediverted nitrogen being conducted to the compressor of the cycle.However, the aforementioned difficulties of the heat exchange for airare greater for nitrogen so that an additional heat exchange,cycle-nitrogen and expanded nitrogen, must be provided. The powerrequirements of the conventional process employing a nitrogen cycle arestill greater than those of the process without a nitrogen cycle.

It is an object of the present invention to provide an improved methodfor fractionating air by liquefaction and rectification which methodavoids the difficulties involved in the aforedescribed conventionalmethods.

It has been explained supra that there is an optimal pressure for theheat exchange between, air and oxygen, at which pressure a minimum ofexcess air is required. This pressure is about 28 atmospheres absoluteat a pressure of the oxygen of 12 atmospheres absolute and it is about80 atmospheres absolute at a pressure of the oxygen of 25 atmospheresabsolute. There is also an optimal air pressure for the unavoidable heatexchange between highly compressed air and nitrogen having no pressure,this optimal pressure being much higher than that for the heat exchangebetween air and oxygen. The reason for this difference is that in thecase of heat exchange between air and oxygen the mean specific heat ofthe air must be as great as possible over a wide temperature range,because not only the sensitive. heat but also the heat of evaporationmust be removed from the oxygen, whereas in the case of the heatexchange between air and nitrogen, in which merely a gas must be heated,the specific heat of the air at the temperature of the environment mustbe as great as possible.

In the process according to the invention a mean pressure is employedfor the heat exchange between air and Oxygen, which pressure depends onthat of the oxygen, and a relatively small amount of highly compressedair, for example at 200 atmospheres absolute, is used in addition to arelatively great amount of air compressed to 5 to 6 atmospheres aboveatmospheric pressure, for the heat exchange with nitrogen, the amount ofhigh compressed air being just sufficient to make up for the deficiencyof air which would otherwise exist. If the pressure of the oxygen is 25atmospheres absolute, 53% of the air would have to be compressed to 80atmospheres absolute, if no additional highly compressed air is used. Ifa portion of the air is highly compressed for the heat exchange Withoxygen of 25 atmospheres absolute, only 24% of the air need becompressed to 80 atmospheres absolute and 2.5% would have to becompressed to 200 atmospheres absolute. The energy required in thesecond case is 23% less than in the first case.

Considerable savings can also be obtained, if the ideas underlying thepresent invention are applied to the closed cycle system, in which a gasis circulated Whose thermodynamic characteristics, particularly vaporpressure and heat of evaporation, are similar to those of oxygen andwhich gas can be compressed in compressors lubricated by oil. Such a gasis argon. Its boiling point is at a pressure of 760 mm. at 87.5 Kelvin(oxygen 90.2 Kelvin) and its heat of evaporation is 1500 kg.-cal./mole(oxygen 1595 kg.-cal./mole). Its only disadvantage seems to be its muchlower specific heat of 5.00 kg.- cal./ C. mole (oxygen 7.01 kg.-cal./C.mole). Argon, however, must be compressed substantially less than nitrogen. If the pressure of the oxygen is 25 atm. absolute, the pressure ofargon need by only 33 atm. absolute whereas the pressure of nitrogenmust be 100 atm. absolute. About the same excess amount of compressedargon is required for heat exchange with oxygen as excess air would beneeded, if air were used for the heat exchange; this is primarily due tothe low specific heat of the argon. However, because of this lowspecific heat, there is no deficiency, if heat is exchanged betweenexpanded argon and air, because 7 parts argon can absorb the heat ofonly 5 parts air at 5.5 atm. absolute. The disadvantage is that one mustemploy a closed cycle, which, in contradistinction to nitrogen, does nottake part in the rectification process, requiring additional heatexchange surfaces for the evaporation of the liquefied argon. Argon,however, can be obtained without difiiculty as a by-product whenfractionating the air so that losses due to leakage in the closed cyclecan be continuously replaced.

A better understanding of the invention will be afforded by thefollowing detailed description considered in conjunction with theaccompanying drawing, the one figure of which is a diagrammaticillustration of a system accord ing to the invention, in which partsunessential to the process according to the invention are omitted.

Referring more particularly to the drawing, the air to be fractionatedis compressed in a turbocompressor 11 and conducted into one of tworegenerators 12 and 12 and therefrom into the rectifying apparatus 14after it has been cooled and conducted through an argon evaporator 13which will be described later. The nitrogen, separated in the rectifyingapparatus 14, is conducted to the other of the regenerators 12 and 12'.In each regenerator a pipe coil is provided through Which argon of apressure of about 2 atm. absolute and coming from the evaporator 13 isconducted. This avoids passage of argon through channels which havepreviously received water and separated carbon dioxide. The argon, afterit has been heated to ambient temperature in the regenerators, iscompressed in a compressor 15 to 33 atm. absolute and thereuponconducted into a heat exchanger 16 in which it is cooled and liquefied.Thereupon it is expanded in a valve 17 and returned to the evaporator 13where it is evaporated and where it liquefies a part of the air comingfrom one of the regenerators. The oxygen separated in the device 14 iscompressed by a pump 18 and evaporated and heated by heat exchange withcompressed argon in the heat exchanger 16.

For replenishing the circuit and replacing argon losses, at small argonseparating column 19 is provided whose product passes through the heatexchanger 16 and is compressed in a small auxiliary compressor 20 andpassed into an accumulator 21. From the latter so much argon is passedinto the argon circuit through a reducing valve 22 as is needed toreplace losses. The accumulator 21 may be omitted, if desired.

Since the amount of argon circulating in the closed circuit is smallerthan the amount of nitrogen in a nitrogen circuit at approximately thesame compression ratio, the power requirements are considerably lessthan those of a nitrogen circuit and are not much greater than whenproducing oxygen without pressure and subsequent compression of theoxygen. The amount of argon circulating in the circuit is in the orderof 24% of the amount of air to be decomposed, whereas the amount ofnitrogen circulating in the nitrogen cycle would be 33%.

What is claimed is:

l. A process for fractionating air comprising the steps of compressingthe air to be fractionated, indirectly cooling and thereby liquefying atleast a part of the air by indirect heat exchange with evaporatingargon, separating the oxygen in the air in a rectifier, compressing theseparated liquid oxygen, evaporating and heating the compressed oxygento ambient temperature by indirect heat exchange with argon circulatingin one closed circuit in which the argon is compressed, the compressedargon being cooled and liquefied by the aforesaid indirect heat exchangewith the compressed liquid and evaporating oxygen, the compressed andcooled argon being expanded and evaporated by the aforesaid heatexchange with the compressed air prior to the compression of the argon.

2. A process as defined in claim 1 in which the compressed air issupplementally cooled in regenerators whose cold storage masses areperiodically cooled by nitrogen separated from the air in the rectifier.

3. A process as defined in claim 1 in which argon lost from the closedargon circuit is replaced by argon separated from the compressed andcooled air by rectification supplementally to the rectification forseparating the oxygen from the air.

4. A process according to claim 3, the supplementally separated argonbeing heated by heat exchange with oxygen, which has been separated fromthe air, and the argon being compressed and expanded prior tointroduction into the closed cycle.

5. A process according to claim 3, the supplementally separated argonbeing heated by heat exchange with the separated oxygen, compressed,accumulated, and expanded prior to introduction into the closed cycle.

References Cited in the file of this patent UNITED STATES PATENTS2,423,273 Van Nuys July 1, 1947 2,657,541 Schilling Nov. 3, 1953 FOREIGNPATENTS 985,083 France July 13, 1951

1. A PROCESS FOR FRACTIONATING AIR COMPRISING THE STEPS OF COMPRESSINGTHE AIR TO BE FRACTIONATED, INDIRECTLY COOLING AND THEREBY LIQUEFYING ATLEAST A PART OF THE AIR BY INDIRECT HEAT EXCHANGE WITH EVAPORATINGARGON, SEPARATING THE OXYGEN IN THE AIR IN A RECTIFIER, COMPRESSING THESEPARATED LIQUID OXYGEN, EVAPORATING AND HEATING THE COMPRESSED OXYGENTO AMBIENT TEMERATURE BY INDIRECT HEAT EXCHANGE WITH ARGON CIRCULATINGIN ONE CLOSED CIRCUIT IN WHICH THE ARGON IS COMPRESSED, THE COMPRESSEDARGON BEING COOLED AND LIQUEFIED BY THE AFORESAID INDIRECT HEAT EXCHANGEWITH THE COMPRESSED LIQUIDAND EVAPORATING OXYGEN, THE COMPRESSED ANDCOOLED ARGON BEING EXPANDED AND EVAPORATED BY THE AFORESAID HEATEXCHANGE WITH THE COMPRESSED AIR PRIOR TO THE COMPRESSION OF THE ARGON.